Cesium mixtures and use thereof

ABSTRACT

Cesium mixtures are produced based on the use of a mixture of at least one reducing agent and at least one cesium compound selected among molybdate, tungstate, niobate, tantalate, silicate and zirconate. The cesium mixtures are useful in the production of OLED-type screens.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 10/465,004, filed Jun. 19, 2003, which was published on Jan. 1, 2004as US 2004/0001916 A1, which is a continuation of InternationalApplication No. PCT/IT02/00301, filed May 7, 2002, which was publishedin the English language on Nov. 21, 2002 under International PublicationNo. WO 02/093664, and the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to cesium mixtures and a processfor using them.

[0003] Cesium has been used for a long time in the electronics field. Inparticular, this metal has been used in the past for the production ofphotosensible surfaces, for example of image intensifiers orphotomultiplier tubes.

[0004] A novel application field for cesium is in the OLED (OrganicLight Emitting Display) screens.

[0005] In short, an OLED is formed of a first transparent planar support(of glass or plastics); a second, not necessarily transparent supportthat may be realized in glass, metal or plastics, essentially planar andparallel to the first support and secured along the perimeter thereof,so as to form a closed space; and a structure in the space that isactive in forming an image. The active structure is formed in turn by afirst set of linear and reciprocally parallel, transparent electrodes,deposited on the first support; a multilayer of differentelectroluminescent organic materials comprising at least one layer ofelectron-conductive material and one layer of an electronic vacancies(also defined in the field as “holes”) conductive material deposited onthe first set of electrodes; a second set of linear and reciprocallyparallel electrodes that are orthogonally oriented with respect to thoseof the first set and in contact with the opposite side of the multilayerof organic materials, so that the latter is comprised between the twosets of electrodes. For a more detailed description of the structure andoperation of OLED screens one can refer for instance to Europeanpublished patent applications EP-A-0 845 924 and EP-A-0 949 696,Japanese published patent application JP-A-9-078058, and U.S. Pat. No.6,013,384. Recently it has been ascertained that the doping of one ormore layers of the organic multilayer with small amounts ofelectron-donor metals, in particular cesium, enables reduction of thepotential difference to be applied to the sets of electrodes for thefunctioning of screens, and thus reduction of the energy consumption ofthe latter.

[0006] Due to its high reactivity to atmospheric gases and moisture,cesium is not usually used in industry as pure metal, but rather in theform of its compounds which are stable to air at room temperature.

[0007] Some cesium compounds release the metal by simple heating. Amongthese compounds alloys with silicon or germanium may be cited, asdescribed for example in European published patent application EP-A-0360 317 and U.S. Pat. No. 5,066,888, as well as the intercalationcompound of cesium with graphite, having the formula CsC₈, cited inEuropean patent application EP-A-0 130 803. These compounds have,however, no practical application at industrial level.

[0008] Cesium dichromate, Cs₂Cr₂O₇, or more commonly cesium chromate,Cs₂CrO₄, is normally used in industry in mixture with a reducing agent.By heating these mixtures at temperatures generally over 500° C., andusually between 550 and 650° C., a reaction takes place wherein chromiumis reduced to a lower valence, as a consequence of which cesium isreleased in vapor form. As reducing agents aluminum, silicon or getteralloys, i.e., alloys based on titanium or zirconium with aluminum or oneor more transition elements, are generally used. The use of thesemixtures is described for example in U.S. Pat. No. 2,117,735.

[0009] These compounds are generally introduced into suitable dispensersthat are able to retain solid particles of the compounds, but have atleast a portion of the surface permeable to cesium vapors. Various formsof dispensers are the object, for instance, of U.S. Pat. Nos. 3,578,834;3,579,459; 3,598,384; 3,636,302; 3,663,121; and 4,233,936. A furtherproperty required for cesium dispensers is not to release gases that aredetrimental to the operation of devices where cesium is used duringtheir production.

[0010] Cesium chromate and dichromate suffer, however, the disadvantageof containing hexavalent chromium, which may cause irritations bycontact, ingestion or respiration and may be carcinogenic in case ofprotracted exposures.

[0011] In production processes of common devices in which cesium is used(image intensifiers or photomultipliers) high temperatures are reached,and only by using chromate and dichromate can one avoid the release ofcesium in an early stage of the process. In addition, limitedproductions are obtained in these cases, and consequently the amounts ofchromate that are used are limited too.

[0012] The temperatures of production processes of OLEDs are lower incontrast. For these screens productions on a very large scale, on theorder of tens of millions of pieces per year, are foreseen. With theseproduction volumes, the safety problems linked with the transportationand use of chromates become significant. In the production of OLEDs itis thus possible and highly desirable not to resort to the use ofCs₂CrO₄ or Cs₂Cr₂O₇ to evaporate cesium.

BRIEF SUMMARY OF THE INVENTION

[0013] An object of the present invention is to provide cesium mixturesparticularly suitable for the production of OLED screens, wherein cesiumis not present in the form of a chromium salt.

[0014] A further object of this invention is to provide a process forthe use of cesium mixtures in the production of OLED screens.

[0015] These objects are achieved by the present invention, wherein afirst aspect relates to cesium dispensers formed by a container able toretain solid particles, but having at least a part of the surfacepermeable to cesium vapors and containing a mixture of at least onecesium compound and at least one reducing agent, characterized in thatthe cesium compound is selected among molybdate, tungstate, niobate,tantalate, silicate, and zirconate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0016] The foregoing summary, as well as the following detaileddescription of the invention, will be better understood when read inconjunction with the appended drawings. For the purpose of illustratingthe invention, there are shown in the drawings embodiments which arepresently preferred. It should be understood, however, that theinvention is not limited to the precise arrangements andinstrumentalities shown. In the drawings:

[0017]FIG. 1 is a perspective view of a first possible cesium dispenseraccording to the invention;

[0018]FIG. 2 is a sectional view along line II-II′ in FIG. 1 of the samedispenser;

[0019]FIG. 3 is a perspective, partially cut-away view of anotherpossible dispenser of the invention;

[0020]FIG. 4 is a top plan view of a further possible dispenser of theinvention; and

[0021]FIG. 5 is a sectional view along line V-V′ of the dispenser inFIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The inventors have found that mixtures of one or more reducingagents and one or more compounds selected among cesium molybdate,Cs₂MoO₄, cesium tungstate, Cs₂WO₄, cesium niobate, CsNbO₃, cesiumtantalate, CsTaO₃, cesium silicate, Cs₂SiO₃, and cesium zirconate,Cs₂ZrO₃, are compatible with the production process of OLEDs, in thatthey are able to vaporize cesium at lower temperatures than thecorresponding chromates practically without releasing gases potentiallyharmful for OLEDs, mainly comprising water vapor. In particular, thevaporization temperatures of cesium from these mixtures are generallylower than 450° C. These temperatures can easily be reached locally onthe cesium dispenser in the purification chambers of OLEDs.

[0023] The mixtures used in the dispensers of the invention may comprisemore than one cesium compound and more than one reducing element orcompound, but generally a single component of each kind is used.Accordingly, in the following, the singular “compound” or “agent” willbe used, but will be understood to include the possibility of theplural.

[0024] As a reducing agent it is possible to use one of the alreadyknown components used in dispensers based on chromates, such asaluminum, silicon, zirconium or titanium, or alloys containing zirconiumor titanium, for example the alloy having a percent composition byweight Zr 84%-Al 16%, produced and sold by SAES Getters S.p.A. under thetrademark St 101®, or the alloy having a percent composition by weightZr 76.5%-Fe 23.5%, produced and sold by SAES Getters S.p.A. under thetrademark St 198®.

[0025] In order to promote contact between the cesium compound and thereducing agent, these are preferably used in the form of powders. Bothcomponents of the mixture generally have a particle size less than 1 mm,preferably less than 500 μm; even more preferably the particle size iscomprised between about 10 and 125 μm. Powders with grains smaller than10 μm are generally difficult to be treated in production and retainedin the dispenser. Furthermore, in the case of the reducing agent,powders too fine may become pyrophoric, thus causing safety problems inthe production plant. In contrast, with powders having sizes greaterthan those listed, the contact between the mixture components becomesworse, and the reaction causing the release of cesium slows down.

[0026] The weight ratio between the cesium compound and the reducingagent can vary between wide limits. The ratio is preferably comprisedbetween 10:1 and 1:10. The use of a cesium compound in great excess withrespect to the reducing agent does not offer practical advantages. Onthe contrary, mainly when the reducing agent is a getter alloy, such asthe mentioned alloy St 101®, its excess in the mixture may becomeuseful, since the portion not involved in the reaction with the cesiumcompound can carry out an absorbing action on the gases that may becomefree during the reaction.

[0027] The mixture can be used in the form of free powders, or it ispossible to preform pellets of the same. The use of pellets bears theadvantage of further improving the contact between the components of themixture and facilitating the charging operations of the container.

[0028] The container can be realized in every material and shapecompatible with the specific application.

[0029] In particular, with reference to the material, this must bechemically inert with respect to the working atmosphere and thecesium-releasing mixture in the whole temperature range foreseen for theuse, generally between room temperature and about 450° C. In the sametemperature range, the material forming the container must not undergonoticeable physical changes, such as to modify its mechanical resistanceor shape, and must have the lowest possible values of gas emission.Materials having these properties are, for instance, metals or metallicalloys, ceramics, graphite, and boron nitride, BN. The use of metals ispreferred due to their easier workability and formability. A furtheradvantage in using metals, graphite and BN is that the dispenser can beheated up to the cesium vaporization temperature by simple passing ofcurrent through the container walls. Preferred metals and alloys formanufacturing the container are molybdenum, tantalum, tungsten, nickel,steel, and nickel-chromium alloys.

[0030] The shape of the container may be any of those known from thepreviously mentioned U.S. Pat. Nos. 3,578,834; 3,579,459; 3,598,384;3,636,302; 3,663,121; and 4,233,936. Containers of various shapes andmaterials are also commercially available, for example, from theAustrian company Plansee or from the USA company Midwest TungstenService.

[0031] In FIGS. 1 and 2 is represented, in perspective and sectionalviews respectively, a possible dispenser using the mixtures of theinvention. In particular, FIG. 2 is the sectional view of the dispenseralong line II-II′ of FIG. 1. Dispenser 10 is formed of two metal foils11 and 12. A depression 13, obtained for instance by cold forming thefoil is arranged in the central part of foil 12. Foil 11 has, in itscentral zone 14 (bordered by the broken line in FIG. 1), a set of smallthrough holes 15. In the assembled dispenser, zone 14 corresponds todepression 13, which contains a mixture 16 of at least one cesiumcompound and at least one reducing agent according to the invention.Foils 11 and 12 can be secured to each other, outside depression 13, inany way assuring powders tightness. For example, a mechanical fixing canbe obtained in the form of “tongues” in a foil folded on the other, or afixing by continuous or spot welding, or combinations of these methods.Finally, dispenser 10 has two lateral projections 17 and 17′ useful forthe handling with mechanical means in the production line and forconnection to electric terminals in order to heat it.

[0032] In FIG. 3 another possible dispenser 30 according to theinvention is represented partially cut away. In this case the containerof mixture 16 is formed by a foil 31 (for instance metallic) similar tofoil 12 of FIGS. 1 and 2, whereas the surface portion of the containerpermeable to cesium vapors is formed by a porous body 32 comprising orformed of a getter material. Body 32 can be maintained in position byany method. For example only, in FIG. 3 a retaining element 33 is shownsecured to foil 31 by means of welding spots 34. Body 32 can bemaintained in the desired position by means of any other retainingelement, fixed to foil 31 in any suitable way. Body 32 may be formed ofsintered getter material only. Such a getter body can be obtained forexample according to the method described in European Patent EP-B-0 719609 in the name of SAES Getters S.p.A. Alternatively, body 32 may beformed of getter material deposited according to various methods on asupporting open structure, such as a wire net having meshes of suitabledimensions. Similar open structures are described for example in U.S.Pat. No. 4,146,497 in the name of SAES Getters S.p.A., or may beproduced by depositing getter particles on a wire net through theelectrophoretic technique, as described for example in U.S. Pat. No.4,628,198. By this structure, the getter body 32 fulfils the doublefunction of allowing the passage of cesium vapors, yet retaining theparticles of mixture 16, and avoiding the pollution of the atmosphere inthe process chamber, where the dispenser is used, by gases such aswater, carbon oxides, etc., that can be released by the components ofthe mixture.

[0033] Finally, FIGS. 4 and 5 represent a further possible form ofdispenser using the mixtures of the invention, that is useful when it isnecessary to vaporize small amounts of cesium. This dispenser has thestructure described in U.S. Pat. No. 3,598,384. FIG. 4 shows thedispenser in a top plan view, and FIG. 5 is a sectional view along lineV-V′ of FIG. 4. Dispenser 40 is formed of a container 41 having anextended structure with a trapezoidal section and a longitudinal slit 42blocked by a wire 43 that allows the vaporization of cesium, butprevents the powdery mixture 44 from escaping. Container 41 is taperedat its ends about two terminals 45, 45′ that fulfill the double functionof closing the ends and as electric terminals for heating the assembly.

[0034] In a second aspect, the invention relates to a process for usingthe above-described dispensers in the production of screens of the OLEDtype.

[0035] The structure of an OLED (briefly described above) is produced,by means of techniques that are typical in the microelectronic industry,by prearranging the first transparent support and depositing in sequencethereon the various forming layers. The electrodes are generallydeposited according to techniques such as screen printing. The layers oforganic materials are obtained in general by means of vaporization orthe technique known as “spin coating,” consisting in the deposition of adrop of liquid material on the support and swift rotation of the latter.

[0036] As the used organic materials and especially the second set ofelectrodes (generally made of metals, such as barium) are extremelysensitive to atmospheric agents and in particular to water vapor, atleast the arranging steps of these layers and the subsequent ones musttake place in proper chambers, under vacuum or inert atmosphere. Thecesium dispensers of the invention are especially suitable forintroducing the element into the active structure during thesetreatments in the chamber.

[0037] In particular the process of the invention comprises the stepsof:

[0038] introducing a cesium dispenser into a chamber having a controlledatmosphere and being provided with means for heating it;

[0039] arranging in the chamber the production intermediate of the OLEDscreen as obtained after having formed the organic multilayer;

[0040] causing the vaporization of cesium from the dispenser by heatingthe latter; and

[0041] carrying out the subsequent production phases of the OLED screenup to its sealing with the second support.

[0042] For the objects of the invention, it is not required that thesephases be carried out in the above order. In addition, the cesiumvaporization operation may be accomplished in different production timesof the OLED. Possible changes in the process of the invention will bedescribed below in more detail.

[0043] The chamber with controlled atmosphere can be one of thosealready provided for carrying out the other production process steps ofthe OLEDs, or can be a chamber dedicated to the cesium vaporizationoperation. This chamber must be provided with means for heating thedispenser, that can be radiative (infrared lamps) or, in the case of adispenser with a metallic or graphitic container, of inductive type. Theheating can be carried out alternatively by direct passage of current,by prearranging a heatable support for the dispenser or even, in thecase of containers made of graphite, boron nitride and metal (forinstance of the type previously described with reference to FIGS. 1 and2), by heating the dispenser through direct passage of current in thecontainer walls. In this latter case, the heating means in the chamberwill be simply electric through means with suitable terminals forconnection to the container.

[0044] In the case where the cesium vaporization chamber is a chamber inwhich other process operations are also effected, the dispenser will beintroduced prior to the vaporization phase and thermally activated at aconvenient time. If, on the contrary, the chamber is used exclusivelyfor vaporizing cesium, in the chamber where the dispenser is alreadypresent there is introduced a production intermediate of the OLED.Cesium is then vaporized from the dispenser by heating it with thepreviously mentioned means at a temperature between about 250 and 450°C., according to the specific cesium compound used therein.

[0045] The vaporization of cesium can be effected in variousintermediate production phases of the OLED. For example, the productionof the OLED may comprise the following main operations:

[0046] production of the first set of electrodes on the firsttransparent support;

[0047] production of the organic multilayer on the first set ofelectrodes;

[0048] vaporization of cesium on the organic multilayer;

[0049] production of the second set of electrodes on the organicmultilayer;

[0050] other possible operations and sealing along the perimeters of thefirst and second supports.

[0051] Alternatively, the cesium vaporization operation can besubsequent to the production of the second set of electrodes.

[0052] The invention will be further illustrated in the followingexamples relating to some cesium compounds of the invention, and inparticular the molybdate and tungstate, in addition to a comparisonexample with the chromate of the prior art.

EXAMPLE 1

[0053] A cesium dispenser is produced, wherein as a compound of theelement the molybdate, Cs₂MoO₄, is used.

[0054] The dispenser is of the kind described in the specification withreference to FIGS. 4 and 5. Container, wire and terminals are made innickel-chromium alloy. The portion filled with the mixture has a sectionof about 1 mm×1.5 mm and a length of 25 mm. This container is filledwith a mixture of one part by weight of cesium molybdate in powder formand five parts by weight of the mentioned St 101® alloy. The powdershave a particle size between 10 and 125 μm. The linear charging of thecontainer is of about 40 mg of mixture per centimeter.

[0055] The so produced dispenser is sample 1.

EXAMPLE 2

[0056] A cesium dispenser is produced as described in Example 1 byusing, however, as a cesium compound the tungstate, Cs₂WO₄. Thisdispenser is sample 2.

(COMPARATIVE) EXAMPLE 3

[0057] A cesium dispenser is produced as described in Example 1, forcomparison, by using as a cesium compound the chromate, Cs₂CrO₄. Thisdispenser is sample 3.

EXAMPLE 4

[0058] This example relates to the cesium vaporization tests from thedispensers produced in the previous Examples 1-3.

[0059] Samples 1-3 are assembled within a chamber that is thenevacuated, are connected to electric loops and fed by a currentgenerator. The current is gradually increased with a slope of 0.1 A/min.The temperature of the sample is measured by a thermocouple welded onthe outer wall of the container, and the current value is registeredthat produces the start of cesium vaporization, taken by the aid of atriode sensor mounted near the vaporization slit. The thus takentemperature values of vaporization start are given in the followingtable. Cs compound Temperature of vaporization start (° C.) Cs₂MoO₄(invention) 295 Cs₂WO₄ (invention) 250 Cs₂CrO₄ (comparison) 625

[0060] Based on the test results, it stands out that mixtures using thecompounds of the invention are able to release cesium vapors in current,and thus temperature, conditions that are lower in comparison to cesiumchromate.

[0061] It will be appreciated by those skilled in the art that changescould be made to the embodiments described above without departing fromthe broad inventive concept thereof. It is understood, therefore, thatthis invention is not limited to the particular embodiments disclosed,but it is intended to cover modifications within the spirit and scope ofthe present invention as defined by the appended claims.

We claim:
 1. A mixture of materials comprising at least one cesium compound and at least one reducing agent, wherein the cesium compound is selected from the group consisting of molybdate, tungstate, niobate, tantalate, silicate, and zirconate.
 2. The mixture according to claim 1, wherein the mixture comprises a single cesium compound and a single reducing agent.
 3. The mixture according to claim 1, wherein the reducing agent is selected from the group consisting of aluminum, silicon, zirconium, titanium, and alloys containing zirconium or titanium.
 4. The mixture according to claim 3, wherein the reducing agent comprises an alloy having a percent composition by weight Zr 84%-Al 16%.
 5. The mixture according to claim 3, wherein the reducing agent comprises an alloy having a percent composition by weight Zr 76.5%-Fe 23.5%.
 6. The mixture according to claim 1, wherein materials forming the mixture are in powder form.
 7. The mixture according to claim 6, wherein the powders comprise particles having a size smaller than 1 mm.
 8. The mixture according to claim 7, wherein the powders comprise particles having a size smaller than 500 μm.
 9. The mixture according to claim 8, wherein the powders comprise particles having a size between 10 and 125 μm.
 10. The mixture according to claim 1, wherein the weight ratio of the materials forming the mixture is from 10:1 to 1:10.
 11. A process for production of an OLED screen comprising a first transparent support, a first set of electrodes, an organic multilayer, a second set of electrodes and a second support, comprising the steps of: introducing a cesium mixture according to claim 1 into a chamber having a controlled atmosphere and means for heating the chamber; arranging in the chamber a production intermediate of the OLED screen, the production intermediate being one obtained after having formed the organic multilayer; causing vaporization of cesium from the mixture by heating the mixture; and carrying out subsequent production steps of the OLED screen up to its sealing with the second support.
 12. The process according to claim 11, comprising the following series of operations: producing the first set of electrodes on the first transparent support; producing the organic multilayer on the first set of electrodes; vaporizing cesium on the organic multilayer; producing the second set of electrodes on the organic multilayer; and sealing the first and second supports along their perimeters.
 13. The process according to claim 11, comprising the following series of operations: producing the first set of electrodes on the first transparent support; producing the organic multilayer on the first set of electrodes; producing the second set of electrodes on the organic multilayer; vaporizing cesium on the second set of electrodes; and sealing the first and second supports along their perimeters. 